If you want to edit icons you can do the following:
Take an icon from an original L2 .utx; you can do this by either exporting all textures from it with umodel (make a command umodel -export icon.utx), or with fixing it with l2wh's utx fixer (doesn't matter much which I think) and then opening it with UTPT.
Once you have the texture you can open/edit it in photoshop (.tga), and once you're done you can export it in .tga, and import it in UnrealEditor, making a new utx there. Put as type DXT1, and name your utx and texture something you can remember.
Then to fix in the custom icon, you need to look for what you want to use it: weapongrp.dat, armorgrp.dat, skillgrp.dat or something.
If you want to change icons from the game itself, like from actions; you need to export all the textures from icon.utx and make a custom new one (you can easily import multiple textures in UnrealEditor).
About the middle step with editing and exporting with photoshop, see some guides here, and also take tools from here: http://www.maxcheaters.com/forum/index.php?topic=109376.0
Good luck! and don't hesitate to ask